The present invention relates, in general, to polyurethane, and more specifically, to an NCO prepolymer having a high content of 2,4xe2x80x2-diphenylmethane diisocyanate (MDI). The NCO prepolymer of the present invention is particularly useful in making elastomeric coatings, adhesives, sealants and the like.
European Patent No. EP 0,573,206 B1, issued to Barksby, discloses polyurethane prepolymers made by reacting a polyether polyol mixture with a polyisocyanate. The polyol mixture of Barksby includes, a) a polyether diol, and b) a polyether polyol having 3 or more hydroxyl groups.
U.S. Pat. No. 5,663,272, issued to Slack et al., discloses allophanate-modified methylene diphenyl diisocyanate (MDI) which is produced by reacting a monoisocyanate and an organic material having at least two hydroxyl groups and a molecular weight of from about 60 to about 6,000 to form a urethane. The urethane is reacted with an isomeric mixture of MDI in an amount such that the product isocyanate has an NCO content of from about 12 to about 30%. The isomeric mixture of MDI is composed of 4,4xe2x80x2-MDI containing from 0 to about 60% by weight 2,4xe2x80x2-MDI and less than 6% by weight of the 2,2xe2x80x2-MDI. The allophanate-modified MDI of Slack is reacted with an organic isocyanate-reactive material to produce an allophanate-modified MDI prepolymer containing urethane, urea and/or biuret groups having an NCO content which is generally from about 5 to about 29% by weight.
U.S. Pat. No. 5,677,413, issued to Barksby et al., discloses polyurethane elastomers prepared from ultra-low unsaturation polyoxypropylene polyols containing up to 20 weight percent internal random oxyethylene moieties. The internal polyoxyethylene moiety-containing polyoxypropylene polyols of Barksby are used to prepare ultra-low unsaturation polyoxyethylene capped polyols, which are haze-free, and may be used to prepare haze-free 4,4xe2x80x2-MDI prepolymers.
U.S. Pat. No. 5,821,316 issued to Quay et al., discloses polyurethane prepolymers made from toluene diisocyanate (TDI) and a blend of two polyols, a) a polyether polyol of 350-2000 equivalent weight and, b) a low molecular weight polyol of 62 to  less than 300 molecular weight, in an equivalent ratio of low molecular weight polyol to polyether polyol of 0.25-2.7:1. The prepolymers are used to produce elastomers that are said by Quay to have good dynamic properties.
U.S. Pat. No. 6,368,714 issued to Robertson et al., discloses moisture-activated adhesive compositions made from 2,4xe2x80x2-MDI. Robertson states that the polyisocyanates used in the invention should have a 2,4xe2x80x2-MDI content of less than 18.0%, preferably less than 10% and most preferably less than 5%.
As those skilled in the art are aware, in general the 2,4xe2x80x2-MDI isomer has a slower reactivity and good liquidity at low temperatures, but yields products which possess poor tensile and tear strengths. Products made from the 4,4xe2x80x2-MDI isomer have good tensile and tear strengths, but its fast reactivity and poor liquidity at low temperatures limit use of the 4,4xe2x80x2-MDI isomer.
Therefore, a need exists in the art for an NCO prepolymer having the low temperature liquidity and slower reactivity of 2,4xe2x80x2-MDI based prepolymers which is capable of producing a finished product with the better tensile strength and tear strength seen in articles made from 4,4xe2x80x2-MDI prepolymers.
Accordingly, the present invention reduces or eliminates the problems inherent in the art by providing an NCO prepolymer which combines the low temperature liquidity and slower reactivity of a 2,4xe2x80x2-MDI prepolymer with the ability to produce a product having the better tensile and tear strength characteristics as is seen in articles made from a 4,4xe2x80x2-MDI prepolymer.
Elastomeric coatings, adhesives, sealants and the like may be prepared from the NCO prepolymers of the present invention.
These and other advantages and benefits of the present invention will be apparent from the Detailed Description of the Invention herein below.